1. Industrial Field
The present invention relates to a D.C. biasing apparatus in impedance measurement, and more particularly to an apparatus for applying a D.C. bias current in inductance measurement.
2. Prior Art and Problems thereof
An inductor has a different inductance value depending on a D.C. bias current because of causes such as saturation of a magnetic core. Accordingly, when inductance is measured under a similar condition as the actual employment state of the device, a D.C. bias power source is used together with an ordinary inductance meter. A problem existing in such measurement is that an error is produced in impedance measurement because an A.C. measuring signal undesirably flows through the D.C. bias power source which is connected in parallel to the device under test.
Here, a conventional method for measuring inductance under D.C. bias will be explained with reference to the drawings. First, one of conventional examples is to connect an inductor 74 having sufficiently large inductance as compared with the device under test between a bias power source and the device under test as shown in FIG. 7 (a). An impedance measuring equipment 71 is a general measuring instrument with which a value of a device under test is determined by applying an A.C. current to a device under test 72 from terminals H.sub.c and L.sub.c, measuring A.C. voltages at terminals H.sub.p and L.sub.d and obtaining a vector ratio thereof. Here, a current i.sub.ac from the terminal H.sub.c is divided at the terminal H.sub.d flowing in the device under test 72 and a current i.sub.e flowing to the bias power source. The shunt ratio at this time is determined principally by the ratio of the impedance of the device under test 72 to the inductor 74. The current i.sub.e flows into the terminal L.sub.c of the impedance measuring equipment through the capacitor C and the power source E. Since processing is performed in the measuring instrument 71 assuming that a current (i.sub.d +i.sub.e) flew in the device under test 72, an error is produced in a measured value. In order to reduce this error, it is just required to reduce i.sub.e. For such a purpose, it is required to make the inductance of the inductor 74 very large for the device under test. For example, when it is desired to take the measurement within an error of 10% in case the true impedance of the device under test is 1K.OMEGA. (approximately 1.6 H) at the measuring frequency of 100 Hz, the inductor 74 requires 16 H or more which is 10 times as high as the true impedance. Further, a current of the same value as the D.C. current which is desired to be applied to the device under test 72 flows in this inductor 74. Therefore, when the bias current is 10 A for instance, the inductor 74 which has a large inductance value as described above becomes huge physically.
As another method, it is thinkable that a parallel resonance circuit 76 is provided in place of the inductor 74 as shown in FIG. 7 (b). At the resonance frequency, the impedance looking the D.C. bias source from the connecting point of terminals H.sub.c and H.sub.p and the device under test becomes very large. Therefore, highly accurate measurement can be made, but the measurement frequency is limited to the resonance frequency, thus lacking in universality.
Hereupon, there is another problem in the D.C. biasing method. FIG. 8 (a) shows an external appearance of the system. Said system 81 includes an inductance measuring unit 81 and a D.C. bias power source unit 82, and a device under test is connected to a measurement terminal 84 attached directly to or in the vicinity of the power source unit 82 so as to be measured.
However, it is desirable to extend and connect a prober 85 with cables as shown in FIG. 8 (b) in order to improve workability in some measurement environment, but a new problem is caused by extension cables. The bias current flows out of a power source 83 and flows in a bias cable 88A as shown with an arrow mark I.sub.1 and flows further in a bias cable 88B through a device under test of the prober 85 and returns to the power source 83 again as shown with an arrow mark I.sub.2. With this, two problems arise with such a composition. The first problem is that a capacitance is formed between bias cables 88A and 88B. If it is assumed for instance that these two connecting cables are parallel conductors as shown in FIG. 9 (a), the capacity between conductors, viz., the capacity C between bias cables is formularized approximately by the following expression. ##EQU1##
The measured signal of the inductance meter expands the measurement error with the coupling (AC coupling) thereof through this capacity C. For example, the capacity between cables when the bias cables 88A and 88b in the length of 1 (m) are held close to each other (D=2 r) is approximately 40 pF, and this means that, when impedance of 1 K.OMEGA. is measured with the measurement frequency of 1 MHz, a stray impedance of approximately 4 K.OMEGA. is added in parallel, thus producing a measurement error of about 20%. Moreover, since this capacity greatly depends on the distance between cables, two lines of cables 88A and 88B have to be fixed in some configuration in order to improve measurement reproducibility, which makes the workability worse.
The second problem is a magnetic field generated by bias currents I.sub.1 and I.sub.2. The magnetic field produced at a point at a distance a from bias cables is formularized as follows: ##EQU2##
In some cases, such apparatus does not satisfy the product standard because of the reason that magnetic field makes trouble for other instrument. For instance, when a bias current of 20 (A) is applied, the magnetic field produced at a point at a distance of 1 cm from the cables is B=4.times.10.sup.-4 (Wb/m.sup.2) per cable, viz. 4 gauss. Furthermore, the magnetic field of 2 times as high as the case of a line of cable, viz., 8 gauss is produced at the point P shown in FIG. 9.
Against above-mentioned problems, when respective cables 88A and 88B are shielded by a shield 91 as shown in FIG. 9 (b), it is possible to prevent the generation of a capacity between cables, but the magnetic field cannot be prevented from being generated.
Also, if a coaxial cable 93 is used as shown in FIG. 9 (c), the bias current flows in a core wire 93a and a casing 93b in the opposite directions, respectively. Therefore, no magnetic field is produced on an outside of the cable. On the contrary, however, the capacity between the core wire 93a and the casing 93b becomes large.